I think tooltips on the buttons would be helpful. When I first got in I had no idea what each button did. It was definitely a weird non-intuitive thing of having the leeches being useful but also harmful but it soon became obvious when you saw the fading numbers showing what the leeches did when they hit you. I would complain about the balance being a little off. I'm guessing the enemies are randomized but instead, having a steadily increasing difficulty might make the game more approachable to people (like me) that find it really hard. And this is coming from someone that enjoys a good tower defence.
Overall a great game made in 48 hours! Great job! :D